An electric power steering apparatus (EPS) which has a motor control apparatus, and provides a steering system of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque, such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of a duty of a pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft) 2 connected to a handle (a steering wheel) 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack-and-pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque Ts of the handle 1, and a motor 20 for assisting a steering force of the handle (the steering wheel) 1 is connected to the column shaft 2 through the reduction gears 3. The electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a steering assist command value of an assist command (a steering assist command) on the basis of the steering torque Ts detected by the torque sensor 10 and a vehicle speed Vs detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 by means of a voltage control value Vref obtained by performing compensation or the like to the steering assist command value. A steering angle sensor 14 is not indispensable and may not be provided. It is possible to obtain the steering angle θ from a rotational position sensor which is connected to the motor 20.
A controller area network (CAN) 40 to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vs from the CAN. Further, a Non-CAN 41 is also possible to connect to the control unit 30, and the Non-CAN 41 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 40.
In such an electric power steering apparatus, the control unit 30 mainly comprises a CPU (Central Processing Unit) (including an MPU (Micro Processor Unit) and an MCU (Micro Controller Unit))), and general functions performed by programs within the CPU are, for example, shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. The steering torque Ts from the torque sensor 10 and the vehicle speed Vs from the vehicle speed sensor 12 are inputted into a current command value calculating section 31. The current command value calculating section 31 calculates a current command value Iref1 based on the steering torque Ts and the vehicle speed Vs using an assist map or the like. The calculated current command value Iref1 is added with a compensation signal CM for improving characteristics from a compensating section 34 at an adding section 32A. The current command value Iref2 after addition is limited of the maximum value thereof at a current limiting section 33. The current command value Irefm limited of the maximum value is inputted into a subtracting section 32B, whereat a detected motor current value Im is subtracted from the current command value Irefm.
The subtraction result I (=Irefm−Im) at the subtracting section 32B is proportional-integral-controlled (PI-controlled) at a PI-control section 35. The voltage control value Vref obtained by the PI-control is inputted into a PWM-control section 36, whereat a duty thereof is calculated. The motor 20 is PWM-driven by an inverter 37 with a PWM signal calculated the duty. The motor current value Im of the motor 20 is detected by a motor current detection means 38 and is inputted into the subtracting section 32B for the feedback.
The compensating section 34 adds a self-aligning torque (SAT) 34-3 detected or estimated and an inertia compensation value 34-2 at an adding section 34-4. The addition result is further added with a convergence control value 34-1 at an adding section 34-5. The addition result is inputted into the adding section 32A as the compensation signal CM, thereby to improve the control characteristics.
In such an electric power steering apparatus, vehicles that are equipped with an automatic steering control mode such as a parking assist function, an obstacle avoidance function, a lane keeping function and so on, and switch between the automatic steering control mode and a manual steering control mode have emerged in recent years. In the vehicles that are equipped with the parking assist function, an automatic steering control that a target steering angle is set based on data such as a camera (an image) and a distance sensor, and an actual steering angle is controlled to follow-up the target steering angle, is performed.
In the conventional well-known electric power steering apparatus that has a function of the automatic steering mode and the manual steering mode, for example in a parking assist, a back-in parking or a parallel parking is automatically performed by controlling an actuator (a motor) based on a relationship between a traveling distance of the vehicle and a turning steering angle, which is pre-stored. That is, a parking assist apparatus recognizes a parking space from a position sensor such an ultrasonic sensor and outputs a steering angle command value (an angle control command value) to the EPS-side. The EPS position-controls the actual steering angle so as to follow-up the steering angle command value, and then the vehicle is guided into the parking space.
FIG. 3 shows a control system of the electric power steering apparatus having the parking assist mode function. Various data from the camera and the position sensor (the ultrasonic sensor or the like) are inputted into a parking-assist-etc. automatic steering command apparatus 50, a parking-assist steering angle command value θt is inputted into a position control section (an angle control section) 51 in the EPS actuator function via the CAN or the like and a parking-assist performing command is inputted into a parking-assist performing judging section 52 in the EPS actuator function via the CAN or the like. The actual steering angle θr which is a steering angle signal from a steering angle sensor or an external is inputted into the position control section 51 and a judging result of the parking-assist performing judging section 52 is inputted into a torque command value gradual-changing switching section 54. The steering torque Ts of the EPS sensor is inputted into a torque control section 53 in an EPS power assist function and a steering-assist torque command value Tc from the torque control section 53 is inputted into the torque command value gradual-changing switching section 54. A position control torque command value Tp from the position control section 51 is also inputted into the torque command value gradual-changing switching section 54. According to a judging result of the parking-assist performing judging section 52, the steering-assist torque command value Tc and the position control torque command value Tp are switched and s switched result is outputted as the motor torque command value, and the motor is driving-controlled via a current control system.
As well, although the position control section 51 also actually performs a speed control and forms a position/speed control section to input a steering angle speed, the here description is limited to the position control for convenience of the explanation.
In this manner, in a case of a normal power assist, a torque control system is used. Meanwhile, in a case of the automatic driving such as the parking assist, the angle control system (the position control system) is used. When switching between the torque control and the angle control, there are problems that switching is not smoothly performed due to a variation of the control torque and that an unintentional assistance torque is occurred due to the torque variation served as a trigger.
For a conventional system of switching from the angle control (the automatic steering) to the torque control (the manual control), an automatic steering apparatus is disclosed in, for example Japanese Patent No. 3311277 B2 (Patent Document 1). In the automatic steering apparatus of the Patent Document 1, as shown in a broken line of FIG. 4A, in a case that a control amount of the actuator (the motor) decreases when switching from the automatic steering control (the angle control) to the torque control, a steering reaction force which a driver receives from the steering wheel is not sharply varied by linearly decreasing the control amount from a value of the automatic steering control to a value of the torque control for passing a predetermined time t0. As shown in the broken line of FIG. 4B, in a case that the control amount increases when switching from the automatic steering control to the torque control, the steering reaction force which the driver receives from the steering wheel is not quickly varied by linearly increasing the control amount from the value of the automatic steering control to the value of the torque control for passing the predetermined time t0. As a result, it is prevented from generating the excessive turning steering angle due to quickly turning the steering wheel and not obtaining the necessary turning steering angle due to slowly turning the steering wheel.